1. Field of the Invention
The present invention relates to an optical information medium comprising one disc formed of a transparent substrate and having an optical recording layer and a reflecting layer at least on one surface thereof and another disc which is bonded to one disc by an adhesive.
2. Prior Art
A digital video disc (DVD) capable of recording and reproducing information with high-density has been now put into practical use associated with the recent development and practical utilization of laser light having a short wavelength. The optical information medium of this type can adopt a structure for bonding two pieces of discs.
A read-only optical information medium having such a structure for bonding two surfaces of discs has a center hole at the center thereof and a clamping area at the outside thereof. A pair of discs which are bonded to each other include at least one transparent substrate comprising at one surface thereof, an information recording area formed outside the clamping area, pits serving as an information recording means formed on the information recording area, and a reflecting layer formed of a metal film and provided on the pit.
Further, as an optical information medium capable of recording information, a tracking guide is formed on the information recording area of at least one substrate, and the substrate is coated with organic dye, etc. by means of a spin-coating process, to form an optical recording layer, and a reflecting layer formed of a metal film is formed on the optical recording layer. Still further, the entire surfaces of a pair of discs are coated with a reactive curing resin as an adhesive by means of a spin-coating process or a screen printing, and the coated surfaces are laid on top of the other to oppose each other, and two discs are bonded to each other by the adhesive set forth above.
In a WORM (write once, read many times) optical information medium, there is employed a system for coating the transparent substrate with a dye material such as an organic dye with a given thickness to form an optical recording layer, and forming a reflecting film such as a metal film on the optical recording layer.
In such a WORM optical information medium, if the optical recording layer of the disc is coated with an adhesive such as a reactive curing resin by a spin coating process or screen printing, etc., and two discs are bonded to each other by this adhesive, residual stress is generated in the joining boundary between the different materials, whereby the discs are liable to warp and the adhesive surface is liable to flake due to the warpage of the discs. Further, there occurs a problem that when a shock is applied to the discs, the discs are liable to flake, thereby deteriorating the shock resistance. Particularly, the flaking is liable to occur at the edges of the two discs, which causes a problem of brittleness against the shock on the edges. Still further, since the adhesion between the two discs deteriorate, the moisture in the air is liable to enter the discs, which causes a problem of deterioration of the layer with moisture.
If a reaction-sensitive resin is employed as an adhesive layer, the shrinkage involved in the curing occurs, and the stress generated thereby influences the recording layer, thereby distorting the recording layer. As a result, a variation of tracking signal and/or reflectance occurs when the discs are reproduced with a laser beam. The variation rate exceeds 30%, thereby obstructing the record and reproduction of the information.
When the discs are bonded by the adhesive, one surface of one disc is coated with an adhesive, and another surface of another disc is overlaid on one disc, thereafter they are irradiated with light so as to cure the adhesive. When one disc is laid on top of the other, bubbles are liable to enter between the two discs, and hence the stress is liable to occur by bubbles at the time of the bonding of the discs, and the magnitude of the stress increases. Accordingly, the influence of the stress upon the recording layer also increases, and there is a possibility that the optical disc is deformed to the extent to be recognized visually. A vacuum degassing apparatus is employed to prevent bubbles from entering the adhesive layer when the discs are bonded to each other, which makes the apparatus large-scaled, and hence increases the operating process, which causes a problem of deterioration of productivity.